1. Field of the Invention
This invention pertains to the field of rotor controls for a rotary wing aircraft, particularly to a tilt rotor aircraft, one whose rotor plane position changes between substantially horizontal and vertical planes. The invention relates to a device for restraining a stationary ring or swashplate against rotation and for permitting relatively lengthy linear displacement and large tilt angles of the swashplate.
2. Description of the Prior Art
The rotor control system for a helicopter or a tilt rotor aircraft includes a rotor on which blades, capable of producing aerodynamic lift or thrust, are rotatably mounted. The angle of attack of the aerodynamic surface of the blades with respect to the airstream is changed by rotating the blades with respect to a reference pitch axis through control input forces applied by pitch links attached to the blades eccentric of the pitch axis. The opposite end of the pitch links are connected to a rotating ring or swashplate driveably connected to a main rotor shaft. The shaft is driven through sets of meshing gears contained in a casing of a transmission, which reduces the speed of the main rotor shaft in relation to the speed of an engine connected to the transmission input.
In order to tilt the rotor while operating in the helicopter mode, i.e., with the rotor in a substantially horizontal plane, the pitch of each blade is changed individually as it rotates. This is called cyclic pitch. However, in order to change the amount of lift or thrust produced by the main rotor, the pitch of all blades can be changed concurrently and by the same amount. This is called collective pitch.
A control system to produce these effects includes a stationary ring surrounding the main rotor shaft, which ring can be raised, lowered or tilted by action of control servos or actuators. A rotating ring is attached to the stationary ring through bearings that allow relative rotation between the rings and maintain elevation and tilt of the rotating ring and rotor identical with those of the stationary ring. The rotating ring carries pitch links extending to each blade so that elevation and tilt of the rings effects pitch changes at the blades. Elevation of the stationary ring and rotating ring axially along the rotor shaft and tilting of these rings angularly with respect to the rotor shaft are produced by hydraulic actuators or servos, a longitudinal servo and multiple lateral servos, connected to the stationary ring at positions spaced angularly about the axis of the rotor shaft.
Conventionally the stationary ring includes a fourth attachment where a stationary scissors assembly is connected to the ring and to the upper surface of the transmission casing. This scissors permits the stationary ring to raise, lower and tilt according to the effect of the control servos, but the scissors prevents rotation of the ring.
However, where the range of axial movement of the stationary ring is large, a conventional scissors assembly is correspondingly large, heavy and occupies more space than is available to accommodate it. The problem of space limitation is especially acute when collective pitch is low, i.e., when the scissors is retracted or folded, rather than extended.
An alternate device for preventing rotation of the stationary ring includes a long, cantilevered channel having a "C" cross section, extending from the top of the transmission casing to the farthest extremity to which the stationary ring must travel to accommodate axial displacement and tilting. A roller attached to the radially outer surface of the ring moves within the channel and transmits to the channel a force required to restrain the ring against rotation. Due to the long distance between the point where the restraining force is applied to the stationary ring and the reaction to this force on the transmission casing, a large bending moment is developed in the channel. Due to the eccentricity between the shear center of the channel and the point of application of the force, a large torsion load is developed in the channel. These bending and torsion loads operate to deflect and twist the channel and require that the channel be large and heavy in order to minimize deflections and transmit the loads to the base of the channel.